1. Field of the Invention
The present invention relates broadly to the production of float glass, and more particularly to determination of the relative elevations of various components which are critical in the economical production of a continuous float glass ribbon of high quality.
2. Description of the Prior Art
Manufacture of flat glass by the float process conventionally involves delivering molten glass at a controlled rate onto a bath of molten metal which has a greater density than that of glass (such as tin or alloys of tin, for example) and advancing it along the surface of the metal bath under physical and thermal conditions which assure that a layer of molten glass will be established on the bath, that from this layer there will develop a buoyant body of molten glass of stable thickness, and that the buoyant body in ribbon form will be continuously advanced along the bath and sufficiently cooled as it is advanced to permit its removal from the bath by mechanical conveying means.
Above the bath of molten metal, an enclosed head space or plenum chamber is provided to contain the so-called float atmosphere. This atmosphere is generally a non-oxidizing gas (usually a mixture of gases such as nitrogen and hydrogen) maintained under sufficient pressure to prevent contamination by leakage of external atmosphere into the head space.
The ribbon is generally removed over the exit end wall or lip of the tank containing the metal bath on one or more so-called lift out rolls and, as it is removed, it immediately enters an enclosed lehr through which it is carried upon a series of horizontally aligned rolls while its temperature is gradually reduced according to a predetermined pattern. The resulting annealed ribbon is then cut into blanks of the desired size.
Handling of the ribbon as it leaves the metal bath and passes through the adjacent annealing lehr is critical to the optical quality of the finished ribbon. Thus, it is desirable for the partially stiffened ribbon to be raised slightly and pass very closely over the exit end or lip of the bath structure and follow a prescribed trajectory as it is separated from the molten metal and carried by the lift out and the lehr rolls. The float glass facility generally operates continuously for a period of several years once production is begun. In order to establish and maintain this path it is necessary to periodically accurately determine, and make corrections to, the elevation of the exit lip and rolls relative to the molten metal surface.
Heretofore, such elevations have been determined by conventional surveying techniques using a surveyor's level and level rod, with the line of support for the glass ribbon through the lehr being utilized as a reference elevation. Despite the best efforts at alignment during installation of the lehr rolls, the elevations are not exactly the same from one roll to the next and from one end of each roll to the other. The roll elevations were determined from the roll ends or journals outside the lehr. Since there is no way to sight directly from one side of the enclosed structure to the other at floor level, the line must be carried by conventional surveying techniques over the top of the structure or around its ends, a distance of several hundred feet, in order to correlate the elevations on opposite sides. This is time consuming and introduces the possibility of error. With the prior art technique the elevation of the tin surface is determined by merely immersing a probe into the tin to measure its depth from the bath floor, and then adding this measurement to the elevation of the floor. Such a procedure, while satisfactory in the past, does not give the degree of precision which is desirable in developing modern day refinements of the float process.